fix: accept metadata in gateway helper signatures

[kshitijk4poor]

Summary

This fixes the gateway crash reported in #849.

Commit a630ca1 started passing metadata=_thread_metadata into _keep_typing(...), but the shared gateway helper signatures still rejected metadata. That caused an immediate TypeError when the gateway received a message.

This PR:

• updates the shared gateway helper contract so _keep_typing, send_typing, and the base media fallback helpers all accept optional metadata
• forwards metadata through the shared fallback paths so thread/topic routing data is preserved instead of dropped
• aligns the affected platform adapter helper signatures (Telegram, Discord, Slack, WhatsApp, Signal) with that shared contract
• adds regression tests covering _keep_typing(..., metadata=...), base fallback helper forwarding, and the gateway adapter signature contract

Why This Matters

• fixes the Telegram gateway crash from issue [Bug]: TypeError in _keep_typing() crashes Telegram gateway on incoming messages #849
• prevents the same unexpected keyword argument 'metadata' failure from surfacing on adjacent typing/media helper paths
• preserves the thread/topic metadata forwarding introduced by a630ca1

Test Plan

• source .venv/bin/activate && python -m pytest tests/gateway/test_platform_base.py -q
• source .venv/bin/activate && python -m pytest tests/gateway -q
• source .venv/bin/activate && python -m pytest tests/ -q

Fixes #849